1. Technical Art
The present invention relates generally to an organic EL (electroluminescent) device, and more specifically to an inorganic/organic junction structure used for a device comprising an organic compound thin film which emits light at an applied electric field.
2. Background Art
In general, an organic EL device is basically built up of an ITO or other transparent electrode formed on a glass substrate, an organic amine base hole transporting layer laminated on the transparent electrode, an organic light emitting layer formed of a material having electronic conductivity and giving out strong light emission, for instance, an Alq.sup.3 material, and an electrode provided on the organic light emitting layer and formed of a material having a low work function, for instance, an MgAg material.
As reported so far in the art, the device has a structure wherein one or plural organic compound layers are interleaved between a hole injecting electrode and an electron injecting electrode. The organic compound layer has a double- or triple-layer structure.
Examples of the double-layer structure are a structure wherein a hole transporting layer and a light emitting layer are formed between the hole injecting electrode and the electron injecting electrode, and a structure wherein a light emitting layer and an electron transporting layer are formed between the hole injecting electrode and the electron injecting electrode. In an exemplary triple-layer structure, a hole transporting layer, a light emitting layer and an electron transporting layer are provided between the hole injecting electrode and the electron injecting electrode. A single-layer structure wherein a single layer has all functions, too, is reported in conjunction with a polymer or mixture system.
Typical structures of the organic EL device are shown in FIGS. 3 and 4.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15, each made of an organic compound, are formed between a hole injecting electrode 12 provided on a substrate 11 and an electron injecting electrode 13. In this case, the light emitting layer 15 also functions as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15 and an electron transporting layer 16, each made of an organic compound, are formed between a hole injecting electrode 12 provided on a substrate 11 and an electron injecting electrode 13.
A problem common to these organic EL devices is reliability. In principle, an organic EL device comprises a hole injecting electrode and an electron injecting electrode and requires an organic layer for efficient injection and transportation of holes and electrons from between these electrodes. However, these materials are sensitive to damages during device fabrication, and offer a problem in conjunction with an affinity for electrodes. For the electron injecting electrode for the injection of electrons, it is required to use a metal having a low work function. For this reason, there is no choice but to use MgAg, AlLi, etc. for materials. However, these materials are susceptible to oxidation and lack stability, and so are a grave factor responsible for a reduction in the service life of the organic EL device and a reliability problem. A further problem is that the deterioration of an organic thin film is much severer than that of an LED or LD.
Most organic materials are relatively expensive. Otherwise stated, there is a great merit in providing low-cost organic EL device products by substituting a partial constitution film with an inexpensive inorganic material.
Furthermore, the development of a device having higher light emission efficiency, lower driving voltage and lesser power consumption than ever before is strongly desired.
To provide a solution to such problems, methods of taking advantage of merits of both an organic material and an inorganic material have been envisaged. That is, an organic/inorganic semiconductor junction structure wherein an organic hole transporting layer is substituted by an inorganic p-type semiconductor has been contemplated. Such contemplation has been investigated in Japanese Patent No. 2636341, and JP-A's 2-139893, 2-207488 and 6-119973. However, it is still impossible to obtain an organic EL device superior to prior art organic ELs in terms of emission performance and basic device reliability.